<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .container {
      position: relative;
      border: 1px solid #333;
      border-radius: 5px;
      padding: 10px;
    }
    /*三角形箭头*/
    .container-arrow {
      height: 16px;
      width: 16px;
      background-color: #fff;
      border-left: 1px solid #333;
      border-top: 1px solid #333;
      position: absolute;
      left: 32px;
      top: -1px; /*注意这里是-1不是0*/
      transform: translate(50%, -50%) rotate(45deg);
    }
  </style>
  <body>
    <div class="container">
      <div class="container-arrow"></div>
      ......<br />
      ........
    </div>
  </body>
</html>